1. Field of the Invention
The system of the present invention relates to over-pressured coal seams and coal bed methane drilling and completion. More particularly, the present invention relates to a continuous circulating concentric casing system for controlled bottom hole pressure for coal bed methane drilling without the use of weighted drilling fluids containing chemicals utilizing annular friction control and or in conjunction with surface choking to provide the required hydrostatic pressure within the bore hole.
2. General Background
In over-pressured coal (CBM) seams and in circumstances when drilling in the direction perpendicular to the face cleats in the coal seams, which has the highest permeability, but in the lowest borehole stability direction, coal seam permeability is easily damaged by the addition of any chemicals or weighting agents as it becomes necessary to have a fluid in the hole with a higher specific gravity heavier than water. In the prior art, to obtain a specific gravity heavier than water, weighting agents and chemicals have been added to water to obtain a desired hydrostatic weight. What happens in coal is that coal has a unique ability to absorb, and to adsorb a wide variety of chemicals that irreversibly reduce the permeability by as much as 85%.
An objective of the present invention is to eliminate a need to add weighting agents and chemicals. The method of the present invention creates back pressure thru the use of either friction on the return annulus or to choke the return annulus, creating back pressure on the formation, or to use a combination of both to create, thru continuous circulating, an induced higher Equivalent Circulating Density (ECD) on the formation. Thus the formation thinks it has a heavier fluid in the hole but only has water in the annulus. This way formation damage is eliminated and higher pressures are exerted in the wellbore creating a reduced collapse window and reduced wellbore collapse issue.